Upon entering water, the player is unaffected by water. The player moves around freely as if on land (sprint, walk, jump, fall). The player can still drown, but is unable to swim around or float back to the surface. Water still breaks fall damage as far as I can tell.
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I wanted to add to this additionally after spending more time on this version:
1.) It does in fact occur on Singleplayer Survival
2.) It is more prevalent on multiplayer in my experience
3.) Restarting client is not a fix, but was coincidence
4.) It affects all players on the server
Additionally, I am sure there is an open topic for this (did not look) but I want to be sure.. This is present in 18w16a and 18w19a. Water does not seem to "fill" in gaps as if the source block does not flow unless there is a source block above the empty space or if the water is only 1 block deep. The screenshot will show what I mean. Water Flowing Bug

It doesn't always affect all players on the server.
When I first experienced this bug, yesterday, the server's owner was also online (nobody else). I couldn't swim, he could. He confirmed he was in survival mode while testing it. I don't know if it was coincidence that he was the owner, or if there was something different about his status or connection that affected things.

Then my experience on my own server with friends must have been coincidence.
Being server owner or having OP permissions does not affect the issue at hand. I am the server owner and hosting on the PC being used to play so I have a 1ms ping. I did notice that the issue seems to occur when the world/chunks are loading, but I can not confirm this so more input would be appreciated of course. This would make sense due to the world gen changes.

Copying the "world" directory from a server to a single-player instance corrected the swimming for that world. Have not yet tried local to server yet.

Copying a functioning single player world to a server breaks swimming.


Therefore MC-129409 is linked as "related"...

Both the singleplayer world and my multiplayer worlds were brand new, generated by the server launch jar file. While i can confirm transferring worlds does break swimming, this is not what is happening on my worlds.
Further, this issue is present on 18w19b as well, which was supposedly going to be compatible with other worlds from previous snapshots.

@unknown, you can add the affected version by yourself.

18w20a has this issue resolved on both new worlds and transferred worlds.
restarting the client does not solve this issue for me. I cant swim on my multiplayer server but other players are able to swim fine, so it doesn't seem to be affecting everyone.